In recent years, it has been extensively investigated to perform separation and purification of fluid mixtures using selectively permeable membranes in place of processes, such as distillation and condensation, in which changes in phase are involved, consuming a large amount of energy.
Most of the processes for separation and purification of fluid mixtures utilizing membranes that have been put to practical use on a commercial scale are directed to liquid-liquid separation, such as production of pure water from sea water, disposal of factory wastes, and concentration of juices, and liquid-solid separation. Almost no process for gas-gas separation has been put to practical use. One of the reasons for this is that selective permeability is low; i.e., since there is no available membrane which allows a specific gas to pass therethrough, while almost completely preventing another gas from doing so, the production of gas of high purity needs a multistage process in which membrane separation is repeated many times. Therefore, a large-sized equipment is required. Another reason is that because of low gas permeability, it is difficult to process a large amount of gas. Moreover, in general, there has been the tendency that when selective permeability is increased, gas permeability is reduced, whereas when the gas permeability is increased, the selective permeability is reduced.
Various producing processes for membrane have been developed to improve the characteristics of selectively permeability and gas permeability. Typical production method is provided by casting a polymer solution to form an asymmetrical membrane, of which active skin layer has a greatly reduced thickness, as described in U.S. Pat. No. 4,230,463 and another method in which an ultra thin membrane corresponding to the active skin layer is prepared independently and joined together with a porous support to form a composite membrane as described in U.S. Pat. No. 3,767,737. These methods, however, are not necessarily satisfactory for commercial use. The methods are not commercially available because the required polymers or copolymers which are satisfactory in all the respects of selective permeability, permeability, heat resistance, chemical resistance, strength, and so forth are not commercially available.
Accordingly, the present invention is intended to produce those membranes which have satisfactory physical properties, such as selective permeability, permeability, heat resistance, chemical resistance, and strength, by using not only a single material, but by using different materials in combination.